This invention relates to a printing method of forming an image directly on a printing medium.
Printing methods for forming an image on a printing medium based on image data signals include an electrophotographic system, a sublimating dye transfer or thermofusible transfer system, and an ink jet system. An electrophotographic system involves a process of forming an electrostatic latent image on a photoreceptor (drum) by electrification and exposure, which requires a complicated system, making the apparatus expensive. The apparatus for a thermal transfer system is inexpensive but has a high running cost and produces waste because of use of an ink ribbon. On the other hand, an ink jet printer is inexpensive and has a low running cost with efficient use of ink because ink is directly ejected only to necessary areas of a printing medium.
The ink jet system includes a piezoelectric system, a thermal jet system, an electrostatic system, and a Spark jet system as described, e.g., The Society of Electrophotography of Japan (ed.), Imaging, Part 2, Latest Hard Copy Printing Technology, ch. 3, Shashin Kogyo Shuppansha (1988) and Kokado Shiroshi (ed.), Kiroku Kiroku Gijutsu Handbook, Maruzen Co., Ltd. (1992). Applications or combinations of these systems, such as those disclosed in JP-A-10-175300 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-6-23986, JP-A-5-131633, JP-A-10-114073, JP-A-10-34967, JP-A-3-104650, and JP-A-8-300803, are also used suitably.
The disadvantages of an ink jet printing method resides in that distortion of dot contour due to feathering or positional deviation of dots readily result in image deficiency or unevenness and that the printing speed is slow because of involvement of mass transfer in printing.
In order to improve the printing speed, it has been attempted to reduce resolution while maintaining the requisite number of tone levels by varying dot size through control of drop volume, ejection time, and the like.
For the purpose of solving the problem ascribed to positional deviation of dots, an image forming system which achieves tone reproduction by varying dot size has been proposed as disclosed, e.g., in JP-A-9-1866. According to this system, an image is segmented into a plurality of blocks, input/output characteristics corresponding to the pixel positions are varied among the blocks. For each of the blocks the density levels of image data are converted to density levels of dots to be recorded, and the position at which dots of a size are to be recorded is varied and dispersed among the blocks. As a result, the image structure is prevented from being perceived with the naked eye so as to suppress generation of a moire pattern.
The above-described conventional technology is, in principle, a kind of pseudo area coverage modulation technology using a multi-level dither method, which has only limited discrete density levels. Its concept consists in that regularity of dot configuration (size and position) is minimized to make the image structure less perceptible with the naked eye and to reduce undesirable moire patterns.
In more detail, R, G and B brightness data are converted into density data of three primary colors of printing, C, M and Y. Black component generation and under color removal are carried out based on the density data to obtain C, M, Y, and Bk (black) data. The C, M and Y data are then subjected to correction processing such as masking and then to tone processing together with the Bk data. In the tone processing the pixels at each position in the above-described block are divided into, for example, odd number lines and even number lines, and different tone conversion table characteristics are applied to each of them.
According to this technique, however, because the characteristics of the tone conversion tables are monotonous, resulting prints unavoidably suffer from graininess where low density recording pixels are formed on a white background. Although the system proposed is used with assumption that printing substrates have stable characteristics, considerations should be given to application to printing substrates that cannot be, in fact, seen as always stable in characteristics. In the case of ink jet printing, the percent change of dot size due to, for example, ink spread or feathering that depends on the humidity and surface roughness of the printing substrate increases with decrease of dot size, which results in a subtle change of the printed image. While feathering hardly occurs as far as elaborated image information, such as data from photographs, is printed on paper for the exclusive use, ink jet printers fail to form high quality images on ordinary printing paper or non-absorbing printing media such as plastic sheets.
Additionally, an ink jet printing system is apt to produce errors of dot positioning. Particularly with small dots, the influence of the errors on dot area unevenness will be exaggerated. Therefore, image unevenness attributed to dot positional deviation is readily allowed to manifest in a low density area made of small dots in a printed image.
An object of the present invention is to provide an ink jet printing method which uses an image formation technique effective for obtaining a high quality image and which is applicable to a printing medium that is not always seen as stable in characteristics.
The present invention provides an ink jet printing method comprising forming an ink image directly on a printing medium according to image data signals and fixing the image to obtain printed matter, in which tone reproduction is based on conversion of density levels of the image data into dot sizes, wherein
tone conversion tables are prepared based on at least five characteristics curves representing the relationship of tone values versus energy for forming recording dots,
at least three characteristics curves are prepared each at a prescribed tone value in the half tone range, each having a converted energy value other than the maximum and minimum values, and
the number of recorded dots the converted energy for which is substantially the minimum is a half or more than a half of the total number of the recorded dots at a tone value that is the least of tone values having a recorded dot the converted energy for which is substantially the maximum.
In preferred embodiments of the printing method, at least three periods are used for sub scan (sub-scanning) for the respective printing colors, and the positions of recording dots are varied for the respective printing colors; or the relationship between a plurality of dots in a unit block and the tone conversion characteristic curve for at least one color varies among blocks. This one color is preferably the one having the lowest density, i.e., yellow.
The present invention also provides an ink jet printing method based on the above-mentioned tone reproduction system, wherein:
monochromatic images of at least two colors have different numbers of elements per unit block, and
the unit blocks for the two or more colors are equal in width and length.
In preferred embodiments of the present invention, monochromatic images of at least two colors have different numbers of elements per unit block, the relationship between a plurality of dots in a unit block and the tone conversion characteristic curve varies among blocks for each of the two or more colors, and the unit blocks of the two or more colors are equal in width and length.
It is preferred that one out of four colors except the above-described three colors is a color having the lowest density, i.e., yellow.
According to the present invention, a continuous gradation of tone is obtained by the image formation method in which at least five tone conversion characteristic curves representing the relationship of tone values versus energy for recording dot formation are prepared for preparing tone conversion tables, and there are at least three converted energy values in addition to the maximum and minimum values at a prescribed tone value in the half tone range. Further, even where recording dots of low density are formed on a white background, image graininess is markedly reduced by setting number of recorded dots the energy for which is substantially the minimum at a half or more than a half of the total number of recorded dots at a tone value that is the least of those having a recorded dot the energy for which is substantially the maximum.
Use of the term xe2x80x9csubstantially the maximumxe2x80x9d with respect to recorded dot forming energy implies that the energy of the characteristic curve at the highest tone value is higher than the energy at any other tone value for every color, which is effective in improving flatness in a solid image area.